Can I use a 2014 iMac (Retina 5K, 27'', ultimo 2014) as a display for new Mac Mini?

No. Neither of those Mac's support Target Display Mode using a cabled connection.

see > Use your iMac as a display with target display mode - Apple Support

If you get a new Mac mini, then you will also need an external display.

see > Connect a display to Mac mini - Apple Support

Plus you'll also need a keyboard and mouse for the Mac mini, if you plan to use or sell what you have with the iMac.

There is a workaround product called Luna Display, by Astropad, that might let you use that 27" iMac as a second display. You will still need a real display for the Mac mini. The Astropad site itself acknowledges that Luna Display has limitations (emphasis is theirs).

Astropad – Luna Display

Astropad Blog – Use Your iMac as a Monitor: A Step-by-Step Guide [2024]

"Like any product, Luna Display does have limitations and it’s not recommended for high-motion content. For example, I wouldn’t recommend using Luna Display for gaming or video editing.

Since Luna Display is squeezing a 5K display through a USB cable, when lots of motion occurs on-screen (like from video playback or moving a window quickly) the screen can temporarily become pixelated before it renders in at full resolution."